1,4-hydroiodination of dienyl alcohols with tmsi to form homoallylic alcohols containing a multisubstituted Z -alkene and application to prins cyclization

Article abstract of DOI:10.1021/acs.orglett.5b00485

A regioselective 1,4-hydroiodination of dienyl alcohols has been developed using trimethylsilyl iodide as Lewis acid and iodide source. A range of homoallylic alcohols containing a multisubstituted Z-alkene was synthesized with good to excellent configurational control. The approach was applied in sequential hydroiodination/Prins cyclization to afford multisubstituted tetrahydropyrans diastereoselectively.

Full text of DOI:10.1021/acs.orglett.5b00485

Letter
pubs.acs.org/OrgLett
1,4-Hydroiodination of Dienyl Alcohols with TMSI To Form
Homoallylic Alcohols Containing a Multisubstituted Z‑Alkene and
Application to Prins Cyclization
Yongjin Xu,† Zhiping Yin,† Xinglong Lin,† Zubao Gan,† Yanyang He,† Lu Gao,*,† and Zhenlei Song*,†,‡
†
Key Laborator‡y of Drug-Targeting of Education Ministry and Department of Medicinal Chemistry, West China School of
Pharmacy, and State Key Laboratory of Biotherapy, West China Hospital, Sichuan University, Chengdu 610041, P. R. China
S
* Supporting Information
ABSTRACT: A regioselective 1,4-hydroiodination of dienyl
alcohols has been developed using trimethylsilyl iodide as
Lewis acid and iodide source. A range of homoallylic alcohols
containing a multisubstituted Z-alkene was synthesized with
good to excellent configurational control. The approach was
applied in sequential hydroiodination/Prins cyclization to afford multisubstituted tetrahydropyrans diastereoselectively.
omoallylic alcohols1 are valuable synthons that have found
widespread use in organic synthesis. One of the most
efficient methods to synthesize these species relies on allylation
of aldehydes and ketones with various allylic organometallic
reagents (Scheme 1).2 While these approaches are powerful, they
poor or moderate regio- and stereoselectivity.5 Here, we describe
our discovery that trimethylsilyl iodide6 can serve as both Lewis
acid and iodide source to promote a regioselective 1,4-
hydroiodination7 of dienyl alcohols 1, giving homoallylic
alcohols 2 containing a multisubstituted Z-alkene8 with good
to excellent configurational control (Scheme 1). We have applied
this approach in sequential hydroiodination/Prins cyclization9 to
afford multisubstituted tetrahydropyrans 3 diastereoselectively.
The model scaffold dienyl alcohol 1a was prepared using a Sn-
promoted Barbier reaction10 of 2-bromoallyl bromide with n-
PrCHO, which was followed by Kumada cross-coupling11 with
vinylmagnesium bromide (72% yield over two steps). Hydro-
iodination of 1a was initially examined using 1.5 equiv of TMSI in
CH2Cl2 at −78 °C. The in situ formed 4a underwent rapid
desilylation during workup to give 4b with a Z/E ratio of 85:15.
No other regioisomers due to 1,2-, 3,4-, or 4,1-hydroiodination
were observed. The labile allyl iodide motif gave 4b low stability.
Therefore, a mild allylic substitution of crude 4b with 3,5-
NO2C6H3CO2H was developed to give 2a as a stable solid in 78%
overall yield (Table 1, entry 1). A wide range of additives was
screened to improve Z-selectivity (entries 2−9). Most gave Z/E
ratios that were similar to, or lower than, the one obtained in the
absence of additive. However, catalyzing the reaction with 0.2
equiv of CuI gave 2a with the highest ratio of 91:9 (entry 10).
While decreasing CuI loading to 0.05 equiv lowered the Z/E ratio
(entry 11), using 1.0 equiv of CuI did not improve the Z-
selectivity (entry 12). TMSI proved to be the optimal iodide
source, as using aq HI instead of TMSI decreased the Z/E ratio to
80:20 with only 15% conversion (entry 13), and using I2 or KI
completely inhibited the reaction (entries 14 and 15). The low
conversion in entry 13 also implies that the 1,4-hydroiodination
should not be promoted simply by HI species. The halogen
source also influenced configurational control and hydro-
halogenation efficiency: TMSBr-promoted hydrobromination
H
Scheme 1. 1,4-Hydroiodination of Dienyl Alcohols with TMSI
(1 to 2) and Sequential Hydroiodination/Prins Cyclization (1
to 3)
are poorly suited for constructing homoallylic alcohols
containing a geometrically defined multisubstituted alkene.
This may be due to the limited availability of the requisite allylic
organometallic reagents or to difficulty controlling regio- and
stereoselectivity during the allylation.
We envisioned that 1,4-disubstitution of 1,3-dienes by a
double-bond shift may be a practical way to achieve this goal.
Previous investigations of disubstitution of 1,3-dienes have
focused on transition metal-catalyzed element−element addi-
tion,3 in which 2 equiv of the same element (Si, B, NR2, OR, etc.)
are incorporated into dienes. In contrast, disubstitutions of
unsymmetric 1,3-dienes with two distinct functional groups4
have been studied in a limited scope due to the difficulties of
regio- and stereocontrol. For example, using highly reactive
electrophiles to initiate 1,4-disubstitution frequently results in
Received: February 15, 2015
Published: March 31, 2015
© 2015 American Chemical Society
1846
DOI: 10.1021/acs.orglett.5b00485
Org. Lett. 2015, 17, 1846−1849

Organic Letters
Letter
a
a
Table 1. Screening of Reaction Conditions
Table 2. Scope of Dienyl Secondary Alcohols
c
d
entry
X source
additive (equiv)
2a (%)
Z/E
1
TMSI
TMSI
TMSI
TMSI
TMSI
TMSI
TMSI
TMSI
TMSI
TMSI
TMSI
TMSI
aq HI
I2
78
70
75
72
71
69
70
74
72
76
74
76
85:15
80:20
80:20
80:20
85:15
67:33
80:20
87:13
89:11
91:9
2
BF3·OEt2 (0.2)
TMSOTf (0.2)
EtAlCl2(0.2)
PtCl2(0.2)
InI3(0.2)
3
4
5
6
7
ZnI2(0.2)
8
AgOTf (0.2)
SnCl2 (0.2)
CuI (0.2)
CuI (0.05)
CuI (1.0)
9
10
11
12
13
14
15
16
17
87:13
89:11
80:20
e
CuI (0.2)
10
CuI (0.2)
N.R.
N.R.
60
KI
CuI (0.2)
TMSBr
TMSCl
CuI (0.2)
77:23
CuI (0.2)
N.R.
a
Reaction conditions: 0.2 mmol of 1a, 0.3 mmol of halogen source in
2.0 mL of CH2Cl2 at −78 °C, 30 min, then 0.4 mmol of 3,5-NO2−
C6H3CO2H and 0.44 mmol of KHCO3 in 8.0 mL of DMF, rt, 3.0 h.
b
a
The Z-configuration of 2a was determined on the basis of NOE
Reaction conditions: 0.4 mmol of 1, 0.6 mmol of TMSI, 0.08 mmol
of CuI in 4.0 mL of CH2Cl2 at −78 °C, 30 min, then 0.8 mmol of 3,5-
NO2−C6H3CO2H and 0.88 mmol of KHCO3 in 16.0 mL of DMF, rt,
c
experiments. Isolated yields after purification by silica gel column
d
1
chromatography. Ratios were determined by H NMR spectroscopy
e
b
of crude 4b. 65% yield based on 15% conversion.
3.0 h. Isolated yields after purification by silica gel column
c
1
chromatography. Ratios were determined by H NMR spectroscopy
of crude hydroiodination product.
of 1a with moderate Z-selectivity (entry 16), while no
hydrochlorination took place with TMSCl (entry 17).
a
Table 3. Scope of Dienyl Tertiary Alcohols
The scope of this approach was tested using various dienyl
secondary alcohols 1a−m. Generally good Z/E ratios were
obtained from hydroiodination of 1a−g containing an
unbranched or branched alkyl group (Table 2, entries 1−7).
For terminal bromide-substituted 1f, cyclization to tetrahydro-
furan occurred during allylic substitution with 3,5-
NO2C6H3CO2H, giving 2f in 66% yield with a Z/E ratio of
94:6 (entry 6). Phenyl-substituted dienyl alcohol 1h and its
analogues 1i and 1j carrying an electron-withdrawing group on
the phenyl ring underwent smooth hydroiodination (entries 8−
10). However, 1k with an electron-donating methoxy group on
the phenyl ring appeared to be an inert substrate (entry 11). The
reaction of 1l containing an allylic alcohol motif provided a
complex mixture (entry 12). In contrast, the reaction of 1m
containing a propargyl alcohol motif led to the desired
hydroiodination product 2m in 70% yield with a Z/E ratio of
93:7 (entry 13).
Hydroiodination of dienyl tertiary alcohols 1n−q showed
distinctly higher Z-selectivity than hydroiodination using dienyl
secondary alcohols, though the yields were lower in some cases
(Table 3). It is possible that dehydroxylation under acidic
conditions to form a tertiary carbocation, which is more stable
than the secondary carbocation, complicated the reaction and
lowered the yield. Interestingly, hydroiodination of dienyl
alcohol 5 with a methyl group substituted at the 3-position
showed reversal of configurational selectivity, giving 6 with an E/
Z ratio of 88:12 (Scheme 2). The overall yield in this reaction was
33%, much lower than in the reaction using 1.
a
Reaction conditions: 0.4 mmol of 1, 0.6 mmol of TMSI, 0.08 mmol
of CuI in 4.0 mL of CH2Cl2 at −78 °C, 30 min, then 0.8 mmol of 3,5-
NO2−C6H3CO2H and 0.88 mmol of KHCO3 in 16.0 mL of DMF, rt,
b
3.0 h. Isolated yields after purification by silica gel column
c
1
chromatography. Ratios were determined by H NMR spectroscopy
of crude hydroiodination product.
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DOI: 10.1021/acs.orglett.5b00485
Org. Lett. 2015, 17, 1846−1849

Organic Letters
Letter
Scheme 2. 1,4-Hydroiodination of Dienyl Alcohol 5 To Form
6
Scheme 4. Sequential Hydroiodination/Prins Cyclization of
1a To Construct Tetrahydropyrans 3
A model analysis to rationalize the configurational control
during 1,4-hydroiodination is outlined in Scheme 3. We propose
Scheme 3. Model Analysis To Explain Differences in
Configurational Control
control at the 3-position and equatorial iodide addition accounts
for the cis-stereocontrol at the 4-position.22
that hydroiodination begins with coordination of the Lewis acidic
Si center to oxygen to form an associated ion pair.12 This Lewis
acid-enhanced Brønsted acidity13 subsequently facilitates intra-
molecular protonation at the 1-position of the diene.14 This may
explain the distinct reactivity shown in entries 8−11 of Table 2,
since the weaker inductive effect of the electron-donating aryl
group in 1k should render the proton less acidic than the
equivalent proton in 1i and 1j, preventing the protonation
required to trigger the entire process.
In the iodination step, the transition state 1-s-Z appears to be
favorable because the iodo anion can internally attack the 4-
position in a pathway involving a least motion pathway15
(Scheme 3). Coordination of copper to the diene may also favor
such an s-Z conformation. The higher Z-selectivity observed for
dienyl tertiary alcohols may be attributed to the Thorpe−Ingold
effect,16 which should bring the ion pair closer to the 4-position
and favor internal iodide transfer. In the 1-s-E transition state, in
contrast, the associated iodo anion lies far from the 4-position. As
a result, iodination can proceed only via an unfavorable external
4-attack to give 2-E as the minor isomer. In the reaction of 5
containing a 3-Me group, the transition state 5-s-Z is less
favorable than transition state 5-s-E because of the increased A1,2
strain17 between the Me and the methylene group at the 2′-
position. Thus, transition state 5-s-E should undergo reaction to
give 6-E as the major isomer. Since this pathway is external, the
iodination efficiency should be low.
In summary, we have shown that trimethylsilyl iodide serves as
Lewis acid and iodide source to promote a regioselective 1,4-
hydroiodination of dienyl alcohols. We have demonstrated that
the approach is useful for synthesizing homoallylic alcohols
containing a multisubstituted Z-alkene with good to excellent
configurational control. Detailed model analysis is presented that
rationalizes the observed regio- and stereoselectivity. The
approach has been applied in sequential hydroiodination/Prins
cyclization to give multisubstituted tetrahydropyrans diaster-
eoselectively. Further applications of this method are under
development.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and spectra data for products. This
material is available free of charge via the Internet at http://pubs.
acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: lugao@scu.edu.cn.
*E-mail: zhenleisong@scu.edu.cn.
Notes
The authors declare no competing financial interest.
To demonstrate its synthetic usefulness, we applied this
approach to a sequential hydroiodination/Prins cyclization
(Scheme 4). After hydroiodination of 1a to generate TMS-
protected Z-homoallylic alcohol 4a, another 2.0 equiv of TMSI
and 0.45 equiv of InI318 were added at −78 °C to promote a Prins
cyclization/iodination with aldehydes.19 Multisubstituted tetra-
hydropyran 3 was obtained from the Z-isomer in good yield with
≥95:5 dr.20 Product stereochemistry was determined on the
basis of NOE experiments on 3i. We assume that Prins
cyclization proceeds via the Alder’s chairlike transition state
TS-7,21 in which the Z-alkene accounts for the trans-stereo-
ACKNOWLEDGMENTS
■
We are grateful for financial support from the NSFC (21172150,
21321061, 21290180), the NCET (12SCU-NCET-12-03), and
the Sichuan University 985 project.
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DOI: 10.1021/acs.orglett.5b00485
Org. Lett. 2015, 17, 1846−1849